A fundamental challenge in cancer treatment is the discovery of compounds that are toxic to cancer cells but not healthy cells. A salient feature of cancer is rapid and unrestricted cell division. The vast majority of traditional chemotherapeutics target rapidly dividing cells by disrupting the cell cycle, causing cell death. Because some healthy tissues require cell division as part of their function, antiproliferative cytotoxins can also kill healthy cells, resulting in severe, dose-limiting side effects. Accordingly, new therapeutic methods and new cellular targets must be identified that better differentiate healthy and cancerous cells. The targets may be present in only a small fraction of cancer patients, thereby allowing for a personalized strategy to treat cancer.
Quinone-containing molecules are frequently cytotoxic and harm cells through one of two mechanisms. Many quinones are conjugate addition acceptors and readily alkylate nucleophilic species such as DNA and cysteine residues. Quinones are also substrates for 1-electron reductases, such as cytochrome P450s, cytochrome b5, xanthine oxidase, and glutathione reductase. Reduction of quinones by these enzymes generates a highly reactive semiquinone that can damage biomolecules directly, or can be oxidized by dissolved oxygen resulting in the formation of an equivalent of superoxide anion radical and the parent quinone. Thus, 1-electron reduction of quinones can catalytically create reactive oxygen species (ROS) that damage the cell.
NAD(P)H quinone oxidoreductase (NQO1, DT diaphorase) is an FAD-dependent 2-electron reductase whose primary function is to protect the cell from cytotoxins, especially quinones. Although generally identified as a cytosolic protein, NQO1 has been identified in subcellular compartments such as the mitochondria and nucleus. By reducing quinones in a 2-electron process, NQO1 bypasses the toxic semiquinone and forms hydroquinones, which are commonly unreactive toward oxygen. Hydroquinones are then conjugated with molecules such as glutathione, glucose, or sulfate, and excreted by the cell. However, some hydroquinone-containing molecules are unstable and react with oxygen in two 1-electron oxidations back to the quinone, generating ROS. The relative stability of hydroquinones toward air oxidation cannot be predicted based on molecular structure and it does not correlate with reduction potential.
NQO1 has attracted much attention as a potential target for the treatment of cancer because it has been shown to be frequently expressed at much higher levels in tumors relative to adjacent healthy tissue, particularly in the case of lung cancer. In addition, NQO1 activity appears to increase during tumor progression. Other than for lung, breast, and colon tissues, relatively little data on the levels of NQO1 in normal tissues have been reported. Whereas low levels of NQO1 are reported in bone marrow and liver cells—two tissues frequently damaged by chemotherapeutics—relatively high levels of NQO1 have been noted in stomach and kidney cells.
The prospect of discovering toxins that are activated, instead of deactivated, by NQO1 has attracted researchers for many years. Such molecules would turn this normally cytoprotective enzyme into a liability for the cell. Two general classes of molecules have been discovered that fit this description: DNA alkylators whose electrophilicity is increased after bioreduction, and redox cycling molecules that generate ROS catalytically after reduction. Examples of such DNA alkylators include Mitomycin C, EO9, and MeDZQ, and examples of such ROS generators include β-lapachone (β-lap) and streptonigrin, the cytotoxic mechanisms of which each involve NQO1-mediated bioreduction. These classes of molecules are composed almost exclusively of quinone-containing compounds.
The concentration of β-lap delivered to cells may induce different forms of cell death, with lower concentrations inducing apoptosis and higher concentrations initiating calcium-dependent necroptosis. In addition to ROS generation in RBCs, the poor aqueous solubility of β-lap necessitates the use of hydroxypropyl-β-cyclodextrin (HPβCD) as a solubility aid, high concentrations of which cause hemolysis of RBCs in vitro. To address the issues of compound instability and damage to RBCs, the Boothman and Gao groups have designed a micellar formulation of β-lap that demonstrates greatly improved PK properties and efficacy in murine tumor models (Blanco, Boothman, Gao et al., Cancer Res. 2010, 70, 3896).
While personalized medicine strategies have produced life-saving anticancer drugs, they affect only a small percentage of cancer patients. Because NQO1 levels are highly elevated in a large number of solid tumors, a treatment that successfully exploits NQO1 levels could benefit a significant fraction of all cancer patients. Thus, new therapeutic methods that exploit elevated NQO1 levels and that can selectively inhibit or kill cancer cells are needed to benefit larger numbers of cancer patients.